1. Field of the Invention
The present invention relates to elastic wave filter devices preferably for use as bandpass filters and duplexers including elastic wave filter devices, and in particular, duplexers and elastic wave filter devices, each configured with a plurality of elastic wave resonators.
2. Description of the Related Art
Hitherto, elastic wave filters are widely used as bandpass filters for use in cellular phones and the like. For example, Japanese Unexamined Patent Application Publication No. 2004-242281 discloses a ladder filter including a plurality of surface acoustic wave resonators, and describes that the ladder filter is provided with a first series arm resonator to which an inductance is connected in parallel and a second series arm resonator to which no inductance is connected. Here, it is assumed that fsr1<fsr2 is satisfied, where fsr1 is a resonant frequency of the first series arm resonator, and fsr2 is a resonant frequency of the second series arm resonator.
Further, Japanese Unexamined Patent Application Publication No. 2011-176746 describes an elastic wave filter in which a series arm resonator to which an inductance is connected in parallel has a lower resonant frequency than a series arm resonator to which no inductance is connected.
As described in Japanese Unexamined Patent Application Publication Nos. 2004-242281 and 2011-176746, the series arm resonator to which an inductance is connected in parallel has a lower resonant frequency than the series arm resonator to which no inductance is connected.
However, there is an issue that loss in a passband increases when Q value of the foregoing inductance decreases. On the other hand, when an inductance having a higher Q value is used, there is another issue that fabrication steps become more complicated and lead to higher cost.